Highly accurate position measuring devices are used for sensing the position of moving objects in a wide variety of machines, such as, for example, machine tools or semiconductor fabrication machines. In particular in the last-mentioned systems, the traversing speeds of the machine components to be positioned relative to each other are becoming increasingly higher, which places correspondingly high demands on the position determination. In some cases, positioning accuracies in the range of a few nanometers are required.
In such machines, it is typically required to perform position determination along a plurality of measurement axes as simultaneously as possible to thereby, for example, determine the position of a moving object in a plurality of spatial degrees of freedom. If position sensing takes place at different points in time in the different measurement axes, this causes errors in the determination of the spatial position of object. For example, at the high traversing speeds encountered, temporal variations in the range of a few nanoseconds in the sensing of positions on a plurality of measurement axes may result in position errors on the order of several nanometers. Such shifts in the position-sensing points in time will hereinafter be also referred to as sampling jitter.
In order to overcome this problem, it is known from EP 1 334 332 B1 to avoid the resulting sampling jitter by generating, in response to a request signal from a machine control unit, a light pulse with which the measuring standard used in an optical position-measuring device is scanned. The point in time of position determination is then precisely defined via the generated light pulse. In this publication, the position-measuring device used is a grating-based optical position-measuring device, and the scanning may be performed using both imaging and interferential scanning methods. Suitable for high-accuracy measurements are, in particular, the aforementioned interferential position-measuring devices, and it is generally also possible to use purely interferometric variants besides the grating-based variants. In such interferential position-measuring devices, a beam emitted by a light source is split into at least two sub-beams, at least one of which impinges on a suitable functional element on the object one or more times. In the case of the grating-based variant, the functional element is a measuring standard; in the case of an interferometric variant, it is a reflector mirror or a retroreflector. Subsequently, the sub-beams are superimposed and interfered at a superposition location and then propagate as at least one resulting signal beam toward an evaluation unit, which generates at least one position-dependent measurement signal from the signal beam.